Characteristics of aircraft structure and usage, including geometry, weight, and purpose determine appropriate structural, kinematic, and positioning characteristics of landing gear. Landing gear positioning is fundamentally based on stability considerations during high-stress operations including landing, takeoff and taxiing, when the aircraft should avoid tip back or tip over. Kinematics relates to behavior of landing gear elements that are used to retract and extend the landing gear. Landing gear kinematics largely concern geometry of landing gear in deployed and retracted positions, in combination with the swept volume consumed during retraction and deployment.
Landing gear design for modern aircraft takes into consideration load distribution, stowage space, stowage proximity, and gear kinematic capability to transfer the gear between stowed and deployed positions. Main landing gear are commonly wing-mounted and body-stowed. Heavy aircraft gross weights demand additional or more durable gear posts and tires to meet reasonable load distribution standards. Increasing wing sweep further complicates landing gear design by reducing space available to stow the gear and complicating gear form and structure.
Some landing gear designs have used joints or bends to “jack knife” a landing gear strut to fit the gear into longitudinally shorter stowage space. The result can be a net space increase in the size of the stowed gear to accommodate linkages to deploy the gear, bend the joints, and lock the braces while maintaining the strut in a fully-extended position.
Some aircraft, for example, passenger transport aircraft, often employ a nose wheel tricycle landing gear configuration that results in a nearly level fuselage and cabin floor when the aircraft is on the ground. The nose wheel tricycle configuration also improves stability during braking and ground maneuvering. During landing, the location of the main landing gear relative to the aircraft center of gravity produces a nose-down pitching moment on touchdown, reducing the aircraft angle of attack and the wing-generated lift. Braking forces acting behind the aircraft center of gravity stabilize the aircraft and enable full brake usage, reducing the landing field length.
One challenge in design and implementation of a nose wheel tricycle landing gear configuration relates to storage of the gear when retracted. Some aircraft, for example the Concorde, use main landing gear that shorten during the retraction process because the gear would otherwise be too long to fit into storage bays. A disadvantage of the shortening gear is the introduction of folding linkages and hinges as points of failure. Reliability concerns mandate the avoidance or elimination of points of failure.